Nigerian forces believe they are zeroing in on Boko Haram, but the Islamic terror group behind a wave of murders and abductions has long tentacles that reach into the Central African nation's neighbors.

In Cameroon, which shares much of Nigeria's northeastern border, officials say militants from the group led by Abubakar Shekau have sneaked in with a tide of refugees, causing a rise in kidnappings, fighting and criminal acts just across the border from Nigeria’s Borno state.

"Right now, we are being infiltrated by Boko Haram,” Col. Didier Badjeck, spokesman for the Cameroon Ministry of Defense, told AllAfrica.com. “The military has decided to strengthen the intelligence system to effectively counter this threat, which seems to be gaining local support.”

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No one knows exactly how many fighters comprise Boko Haram, which one month ago kidnapped more than 300 Christian schoolgirls who Shekau has said will be sold into slavery or as child brides. Estimates range from a few hundred to a few thousand, but experts say that does not include a much wider network of sympathizers, informants and spies placed in the Nigerian government.

University of Massachusetts-Boston Professor Darren Kew estimates that Boko Haram has 300 hard-line fighters, with another 1,000 held in Nigerian prisons and as many as 2,000 more hired guns at its disposal. But, he said, it’s important to keep in mind that Boko Haram is more of a movement than an organization.

“In addition to the hard-liners at the center, there are numerous other factions — including “moderates” and fence-sitters — and part-timers as well, many of whom may be sympathetic to the cause, or may simply be young men for hire when needed,” said Kew, who heads the school's Center for Peace, Democracy, and Development.

Founded in 2002 with the goal of advancing Islamic law, Boko Haram's name loosely translates to "Western education is sinful." It is unclear how popular the group's message is, particularly after its most recent savagery has garnered local and international condemnation. But there is no doubt that the group moves easily across Central Africa's borders, embraced by villagers either out of solidarity or fear.

“The boundaries between Nigeria, Chad, Cameroon, and Niger are simply lines on a piece of paper that were drawn there by the French and English when they were in the region,” said John Campbell, senior fellow for African Studies at the Council on Foreign Relations and former ambassador to Nigeria, told FoxNews.com. “It’s very hard to know the extent of support for Boko Haram in Nigeria itself let alone neighboring countries.”

In recent months, Boko Haram's foothold in northern Cameroon and southern Niger seems to have strengthened, as evidenced by an increase in kidnappings, said Kew.

“As of January it appeared that Boko Haram had been largely contained to [Nigeria's] Borno and Yobe states, but since February they have clearly broken out of their core areas," Kew said.

Boko Haram at one point explored an alliance with Al Qaeda, meeting with the terror group's African leaders in Mali last year, said Mark Schroeder, vice president of Africa Analysis for geopolitical intelligence firm Stratfor. But much of the secretive group's growth has been carefully managed and restricted to nations bordering Nigeria, he said.

“They [Boko Haram] have sought to recruit and raise their ranks,” Schroeder said, “but they have been careful. If they grew in size throughout the region they would likely have a coalition fight against them.”

Cameroon officials, who have long prided themselves on their nation's relative stability in the region, say violent spillover seems to come when the Nigerian forces turn up the heat on Boko Haram. On May 5, more than 30 Boko Haram fighters stormed a military post in Cameroon's Far North Region, killing a security officer and a civilian who was being held in custody. In another attack around the same time, Boko Haram fighters attacked a public market in the Nigerian border town of Gambourou, killing more than 200 and sending 3,000 Nigerian refugees pouring into Cameroon.

"Our problems come from our neighbors," Far North Region Gov. Fonka Awah Augustine told Voice of America. "Each time the Nigerian army attacks, Boko Haram becomes destabilized and they are looking for a safe ground to settle, and each time Boko Haram equally attacks either the army or a particular community, they cause the flow of the population in their thousands into our region.”

Perry Chiaramonte is a reporter for FoxNews.com. Follow him on Twitter at @perrych